Incorporation of labeled glycerol into rat photoreceptor phospholipids was found to follow a circadian rhythm, being elevated during the daylight hours. Autoradiography revealed that the most intensely labeled membranes were in the inner segments, possibly representing precursors of outer segment membranes. In contrast, opsin synthesis, glycosylation, and intracellular transport occurred at a constant rate throughout the diurnal cycle. Thus, opsin appears to be inserted continually into phospholipid vesicles formed predominantly during the daylight hours. Palmitic acid was found to be incorporated into bovine rhodopsin in a covalent, alkali-labile linkage. The incorporation occurred as a late post-translational event, not affected by inhibition of protein synthesis, and possibly as the rhodopsin reached the rod outer segment.